1. Field of the Invention
This invention relates to a hot forming mold which is excellent in heat resistance, machinability and the like, and more particularly to a hot forming mold suitable for hot gas pressure forming and to a method of manufacturing the same.
2. Description of the Prior Art
The hot gas pressure forming method has various advantages, including: (1) under isotopic pressure a molded article having a uniform density may be obtained, (2) a complicated shape of a molded article can be obtained with high accuracy in a single step, (3) the surface of a molded article is beautiful. Therefore, attention has been recently paid to this method in the field of "superplastic forming and diffusion bonding".
The superplastic forming and diffusion bonding method has received much attention as a technique for integral manufacturing of the airframe of an airplane. A superplastic metal sheet such as aluminum alloy, titanium alloy, two-phase stainless steel and the like is subjected to hot-forming and/or diffusion bonding by inert gas pressure at high temperature. The forming molds used must be capable of withstanding a forming temperature of approximately 600.degree. C. for aluminum alloy and of approximately 1,000.degree. C. for titanium alloy and two-phase stainless steel, and must have dimensional precision of less than 0.5%.
In the case of sheet-like prepreg material, for example, such as glass fiber reinforced plastic (GFRP), carbon fiber reinforced plastics (CFRP), and amide fiber reinforced plastics (AFRP), attention has been paid to a so-called autoclave molding method in which these materials are laminated and then molded under gas pressure in the autoclave. In this case, the forming temperature is typically 120.degree. C. to 200.degree. C., and up to 350.degree. C. at the maximum.
Other than the hot gas pressure forming methods described above, there is a hot forming method for glass for automobiles, in which the dimensional precision and surface precision of hot forming molds are required. In this case, the hot forming mold must have heat resistance up to about 800.degree. C. at the maximum and a low coefficient of thermal expansion.
In the molding techniques, the hot forming molds are required to have excellent heat resistance, and to be of a large size, up to on the order of several meters. Conventional heat-resisting molds of heat-resisting alloys, such as austenite stainless steel, Inconel or the like, or molds of heat resisting epoxy resins, are difficult for practical uses, because the expense of manufacturing large ingots of said materials and of machining the ingots is extremely high. Further, harmful gases occur at the time of manufacturing epoxy resin molds, giving rise to deterioration of material characteristics in hot forming molds made therefrom.
In view of their heat resistance, ceramic materials such as alumina, sialon, etc. are excellent materials. But they are extremely difficult to work and to transform into large ingots.